1. Field of the Invention
This invention relates generally to a system for measuring a traveling direction of a vehicle and, more specifically, to a vehicular traveling direction measuring system which can derive an accurate traveling direction of a vehicle by utilizing a geomagnetic sensor and a gyro sensor.
2. Description of the Background Art
One previously proposed system for measuring a vehicle traveling direction is exemplified by Japanese First Patent Publication No. 58-34483.
In this publication, a system utilizing a geomagnetic sensor which provides an absolute traveling direction of the vehicle, and a gyro sensor which provides a relative variation in traveling directions of the vehicle is disclosed. The system is adapted to measure the traveling direction of the vehicle by utilizing the combination of outputs from the geomagnetic sensor and the gyro sensor, as will be described hereinbelow with reference to FIG. 1.
Assuming the vehicle is running in a direction P on a straight road where the geomagnetism environment is unfavorable, a traveling direction derived by the gyro sensor becomes a straight line Q which is stable and close to the direction P because the gyro sensor is, as is known, not affected by environmental geomagnetic conditions. Deviation between the directions P and Q is caused by an error due to drift of the gyro sensor. Since the error is accumulated with a lapse of time or increment of distance travelled by the vehicle, the deviation between the directions P and Q gets larger as shown in FIG. 1. On the other hand, because the direction derived by the geomagnetic sensor is affected by the environmental geomagnetism, it is exemplarily described as a sine curve R in FIG. 1.
Accordingly, when the vehicle traveling direction is measured solely based on the output of the geomagnetic sensor as disclosed, for example, in Japanese First Patent Publication No. 59-100812, an error between the true vehicle traveling direction P and the measured traveling direction R becomes considerably large. On the other hand, when the vehicle traveling direction is measured solely based on the gyro sensor as disclosed, for example, in Japanese First Patent Publication No. 59-202014, an error between the true vehicle traveling direction P and the measured traveling direction Q gets larger with the lapse of time as mentioned above.
In order to overcome the above-mentioned defects, predetermined constant lower and upper threshold levels S1 and S2 are set as shown in FIG. 1, which are selected by deriving difference between the directions R and Q measured by the geomagnetic sensor and the gyro sensor, respectively. When the direction R exceeds the threshold level S1 or S2, the direction R is corrected to be equal to the threshold level S1 or S2 so as to obtain a directional line T which is held as a measured traveling direction of the vehicle. Accordingly, the direction R obtained by the geomagnetic sensor is corrected by the direction Q obtained by the gyro sensor to reduce the error to a difference L between the threshold level and the true traveling direction P of the vehicle.
However, as is clear from FIG. 1, the error L is still considerably large. Therefore, when a geographical position of the vehicle is calculated using the corrected directional line T, a difference between a true position of the vehicle and a calculated position of the vehicle gets larger with the lapse of time because the error is accumulated with the lapse of time.
Further, because the gyro sensor is utilized only for setting the threshold levels to provide a stabler output of the geomagnetic sensor, the most advantageous aspect of the gyro sensor, that is, providing a relative directional variation with high accuracy without being affected by environmental geomagnetism, is not effectively utilized.